Why is the online version of Excel of Office 365 so limited?
In advance it is not limited. For software that is free, the editing options are many.
One could ask exactly why does microsoft give so much for free?
To answer your question and mine very simply, it's about money.
Microsoft gives so much for free, because it expects that the users sometime when they have developed their knowledge in this software, then also take the step to buy it as a full version (depending on the needs).
Here are the options you have with the online version ... and as I said, there are quite a few:)